The output of the element is a function of both voltage and current at the
point it is measured. The element sums 2 voltages, one that represents the
voltage at the point of test and the other that represents the current. When
SWR is 1:1 these 2 voltages are equal so that the voltage output is actually
twice that of what is contributed by voltage alone at the point of test when
measuring forward power. This is why it would be zero when measuring
reverse( the voltage representing the current adds negatively). In the
hundred watt example you used the voltage out to the meter would be 141.4v
not 70.7v

"JGBOYLES" wrote in message
...
Hi, I thought I understood this but recent discussions left me wondering.
A Bird Wattmeter provides a voltage that is proportional to the forward
power
minus the reflected power. This assumes the output impedance is known and
constant, usually 50+-j0. Assume a Bird has a 0-10vdc meter for
indication. I
don't know what it is, but for discussion.
Input 100 watts into a 50 ohm load, and you get 70.7 volts, the Bird
scales
this to 1 volt, and you get 10% deflection on your wattmeter, or 100
watts.
Input 1000 watts and you get 223.6 volts which the Bird scales to 3.16
volts,
or 316 watts. 316 does not equal 1000, so the scale on the meter has to
have
V**2 relationship to indicate 1000 watts. So 1000 watts is 31.6% of full
scale. I thought all Ham wattmeters did this. Of course it is highly
dependent on the Z the wattmeter sees. Wattmeters that actually multiply
V and
I are another subject, it is hard to keep them in line without causing
some
insertion loss, is it not?
73 Gary N4AST